I can't answer that question for you- Its subjective, really. check some samples. I can't compare the two and give a subjective opinion cause I don't have much experience with a stock UA-5, except from SpesulEd's - his tapes sound great, IMO.
but, the more I think about it, the more I'd sway to the UA-5, regardless... if you are unsure, its prolly in your best interest to give yourself the most possibility of upgrading later on.
With the AD-20, if you ever switch mics (which you prolly will - we all do!) - you'll most likely need to get that phantom power supply anyways.